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Abstract

A major limitation to attaining low-loss single-mode guidance in hollow core photonic crystal fibre (PCF) is surface guided modes that are trapped in the core surround. This is particularly severe when high index (n > 2) glasses are used. By modelling a structure that has the characteristic features of a realistic fibre we show that, by tuning the thickness of the core wall, the influence of these ‘surface’ modes can be minimised. For a refractive index of 2.4 we predict power-in-air fractions of over 95% over a fractional bandwidth of ~ 5%, peaking at over 98%. The designs are appropriate for mid- to far-IR PCFs for which suitable glasses (e.g., tellurites and chalcogenides) have high refractive indices.

Figures (6)

Fig. 1. Core design of high-index PCF using geometrical shapes, showing air regions in grey (different shades of which are shown for clarity) and glass in white. For simplicity, only the unique 112 of the design is given; the remainder may be obtained by applying C6v symmetry operations. The two large circles lie on the cladding lattice, and the radius s is chosen such that the corresponding circle touches arc A and line B. The adjustable parameters are the cladding hole radius r and the core wall thickness t, which is controlled by the radius R of the central hole; in this paper we consider a fixed hole radius r = 0.4Λ and vary R.

Fig. 2. (a) An SEM image of a typical silica hollow-core fibre (fabricated by Blaze Photonics), and (b) the model design for high-index glass created using geometrical shapes. The core wall thickness in the model design is t = 0.05Λ.

Fig. 3. Mode trajectories for a range of three core wall thicknesses of the PCF design shown in Figs. 1 and 2. The modes are labelled by symmetry according to the notation of [24]; the key to symmetry types is given in Fig. 4. The fundamental air-guided mode is shown by an arrow in each figure.

Fig. 4. Modes of the PCF structure with core wall thickness t = 0.05Λ. The red shaded regions in the top-left and lower-right corners show the band edges, and the air-line is marked with a vertical black line. The fundamental air-guided mode is shown by an arrow.

Fig. 5. Plots of the axial Poynting vector (normalised to unity over the supercell, and shown on a linear scale) for selected surface modes near to the air-line, at frequencies above and below the ‘clean’ region of Fig. 4. Each row of the figure shows two modes of the same symmetry type. The two modes of each doubly-degenerate pair have been added in intensity to show their structure more clearly.

Fig. 6. Plots of the axial Poynting vector (normalised to unity over the supercell, and shown on a linear scale) for the lowest-order air-guided modes of the PCF structure with core wall thickness t = 0.05Λ at frequency k0Λ = 5.5. The two modes of each doubly-degenerate pair (at βΛ = -0.266,-0.105) have been added in intensity, and the colour scale used is the same as that in Fig. 5.